1. Field of the Invention
The present invention principally relates to an image forming apparatus such as an electrophotographic type or electrostatic printing type copying machine or printer. In particular, the present invention relates to the detection control of the tint and positions of developer images of colors formed in an image forming apparatus.
2. Description of the Related Art
In a color image forming apparatus including a plurality of photosensitive members, misregistration occurs between images of different colors due to, e.g., mechanical mounting errors of the photosensitive members, or optical path length differences or optical path changes of laser beams of different colors. Also, the image density for each color fluctuates due to various conditions such as the usage environment and the number of copies, therefore, the color balance, that is, the tint changes.
In an image forming apparatus, therefore, registration correction and density correction are performed between images of different colors. Japanese Patent Laid-Open No. 11-143171 has proposed a method of detecting and correcting misregistration and detecting and correcting densities by forming a registration detection pattern and density detection pattern on an intermediate transfer belt. Japanese Patent Laid-Open No. 11-143171 avoids increases in size and cost of the apparatus by detecting the registration and density detection patterns by using the same sensor.
Also, Japanese Patent Laid-Open No. 2001-166553 has disclosed a method of forming both registration and density detection patterns on an intermediate transfer belt, and correcting the misregistration and densities by the same sequence, thereby shortening the time required for the correction.
In the related art, the correction control time is shortened by correcting the misregistration and densities by the same sequence. In this method, a plurality of registration detection patterns and a plurality of density detection patterns may be repetitively formed on an intermediate transfer belt, in order to avoid the influence of periodical variations generated by the rotation periods of, for example, photosensitive members or rollers for driving the intermediate transfer belt, because the photosensitive members or rollers are off-centered. To correct misregistration and densities by the same sequence, the detection patterns must be formed within one circumference of the intermediate transfer belt. As image forming apparatuses have been downsized recently, however, the circumference of the intermediate transfer belt shortens. Accordingly, demands have arisen for accurately correcting misregistration and densities even with a short pattern length.